Anisotropy in the dielectric function of Bi$_2$Te$_3$ from first principles: From the UV-visible to the infrared range

TL;DR

This study uses first-principles calculations to analyze the anisotropic dielectric properties of Bi$_2$Te$_3$ across UV-visible and infrared spectra, highlighting the effects of local fields and doping on optical responses.

Contribution

It provides a detailed theoretical analysis of the anisotropic dielectric function of Bi$_2$Te$_3$, including local field effects, phonon properties, and plasmon frequencies, with comparison to experimental spectra.

Findings

Good agreement between theoretical and experimental spectra when including free carrier effects.

Anisotropic plasmon frequencies can help determine doping levels and types.

Local field effects significantly influence UV-visible dielectric properties.

Abstract

The dielectric properties of Bi$_2$Te$_3$, a layered compound crystallizing in a rhombohedral structure, are investigated by means of first-principles calculations at the random phase approximation level. A special attention is devoted to the anisotropy in the dielectric function and to the local field effects that strongly renormalize the optical properties in the UV-visible range when the electric field is polarized along the stacking axis. Furthermore, both the Born effective charges for each atom and the zone center phonon frequencies and eigenvectors needed to describe the dielectric response in the infrared range are computed. Our theoretical near-normal incidence reflectivity spectras in both the UV-visible and infrared range are in fairly good agreement with the experimental spectras, provided that the free carriers Drude contribution arising from defects is included in the infrared response. The anisotropic plasmon frequencies entering the Drude model are computed within the rigid band approximation, suggesting that a measurement of the reflectivity in the infrared range for both polarizations might allow to infer not only the type of doping but also the level of doping.